Flow graph should be rewritten using Specter

[jsa-aerial]

While issue #2 speaks to the need of splitting this off into a separate lib which would then be a dependency, this issue is a marker indicating the need to rewrite the (rather ugly) custom graph rewriting code to use Specter. Specter has vastly cleaner general purpose rewrite code for all manner of nested datastructures, including graphs, trees, maps, and all manner of nesting. This is exactly what the flow graph recursive rewriting code needs.